In general, resins of polyethylene, polypropylene, polyethylene terephthalate and others are used in paper laminates. However, when disposed of after use, they will increase the amount of their wastes. In addition, since they do not almost degrade in the natural environment, they will semi-permanently remain in the ground even after disposed of for land reclamation. Moreover, some plastic wastes are further problematic in that they destroy the scenery and even the living environment of marine organisms.
To solve these problems, polyhydroxycarboxylic acids such as polylactic acid, and aliphatic polyesters derived from aliphatic polyalcohols and aliphatic polycarboxylic acids have been developed these days for biodegradable thermoplastic polymer resins.
These polymers are characterized in that they may 100% biodegrade in animal bodies within a period of from a few months to one year, or when kept wetted in soil or seawater, they begin to degrade in a few weeks and completely degrade within a period of from about one year to a few years, and their degradation products are lactic acid, and carbon dioxide and water all harmless to human bodies.
In particular, the application of polylactic acid is expected to expand since its starting material, L-lactic acid has become mass-produced through inexpensive large-scale fermentation and since the polymer has good properties in that it rapidly degrades in compost and is resistant to fungi and it causes little odorization and discoloration of foods and drinks that contain it.
The following patent publications are referred to herein.
JP-A-4-334448 discloses a technique that relates to a biodegradable composite material and its production method.
Specifically, the composite material of the technique disclosed is produced by coating the surface of a substrate that contains vegetable fibers, with a polylactic acid or its derivative. This is highly resistant to water and oil and is highly biodegradable, and this is favorable for wrapping and packaging paper for foods and drinks and for wrapping and packaging materials for medical use.
The composite material is biodegradable and biocompatible, and when disposed of after use, it is naturally biodegraded by the action of microorganisms in soil and water and therefore does not pollute the natural environment. This is produced by coating the surface of a substrate that contains vegetable fibers, with a polylactic acid or its derivative. The substrate is formed of a material that consists essentially of various vegetable fibers. It includes, for example, paper such as woodfree paper, shoji paper; yarn and rope of cotton, Manila hemp or the like; containers, nets and others formed of them.
Polylactic acid for use herein includes poly-D-lactic acid, poly-L-lactic acid, and poly-D,L-lactic acid; and its derivatives are, for example, polylactic acid-glycolic acid copolymers and polylactic acid-glycerin copolymers.
JP-A-8-290526 discloses a technique that relates to an aluminium-biodegradable plastic laminate. When disposed of in the ground, the laminate readily biodegrades. This is a laminate of an aluminium material and a biodegradable plastic that generates acid when it degrades. The aluminium material includes aluminium foil, aluminium deposit layer, etc. The biodegradable plastic may be in any form of film, adhesive, ink, etc. The biodegradable plastic that generates acid when it degrades includes 3-hydroxybutyric acid-3-hydroxyvaleric acid copolymers, condensates of aliphatic diols and aliphatic dicarboxylic acids, polylactic acid prepared through polymerization of lactic acid, etc. These biodegradable plastics generate their starting acids such as aliphatic dicarboxylic acids or lactic acid. The acid acts on the aluminium material to convert it into aluminium oxide, and the aluminium material is thereby decomposed and lost.
However, the melt tension of lactic acid-based resin such as polylactic acid is low, and the resin melt therefore greatly necks in when extruded out through a die hole in extrusion lamination molding. As a result, the selvage of the resin laminate produced significantly fluctuates and the laminate is often difficult to wind up. Further, thin laminates with the resin are often cut and broken while produced, and, if so, their safe production is impossible.
Moreover, the thickness fluctuation of the resin layer of the paper laminates produced is great, and under ordinary molding conditions in ordinary molding machines, the width of the laminate that is substantially uniform in the cross direction is approximately from 80 to 85% or so of the total laminate width.
Adding a peroxide to polylactic acid is generally known method of increasing the melt tension of the resin. This is for crosslinking the polymer during its extrusion pelletization or extrusion molding (JP-A-10-501560).
The method is surely effective for increasing the melt tension of polymer, but has many industrial problems such as those mentioned below:
<1> The polymer is much crosslinked to give gel, and the films formed of it have many fish eyes and therefore have no commercial value;
<2> The reaction is difficult to control, and the reproducibility of the method to give the intended crosslinked products is not good;
<3> The peroxide requires an additional apparatus for safely handling it.
On the other hand, JP-A-2001-123055 discloses a mixture of polyolefin such as polyethylene or polypropylene generally used for extrusion lamination, and microorganism-degradable thermoplastics or polylactic acid.
However, though an apparent melt tension of the mixture is increased, the mixture has problems such as those mentioned below:
<1> It is impossible to obtain a uniform molding,
<2> A heat-seal strength of the film extremely decreases by lamellar peeling and,
<3> An impact strength of the molding extremely decreases by lamellar peeling.
As in the above, it is substantially impossible to obtain laminates of polylactic acid through conventional extrusion lamination in the current situation in the art.